
1) Given that, we can write out the following:
l=w+9
Perimeter (2P)= 2(w+l)
2) So now, let's plug into the Perimeter equation the given data:

Note that now, we've got the width. So let's plug it back into the first equation to get the length of the rectangle:

Thus, the width is 39 ft and the length is 48 ft